“S” system is used by many European breweries, also known as European Sankey. Distinguished by it’s longer, narrower probe than the D System. It is used by breweries such as Heineken, Becks, Amstel Light, and Pilsner Urquell.
The keg coupler connects to the keg valve, “tapping” the keg. Both the gas line from the CO2 tank / regulator and the beer hose to the beer faucet attach to the coupler.